We have examined the role of CD4 and CD8 in MHC class recognition. In these studies, we have evaluated the ability of CD4 and CD8 molecules to function as signalling molecules for mature and developing T cells. Interestingly, we have found that CD4 function is related to TcR expression and appears to play an important signalling role in T cell differentiation. Indeed, there are a number of indications in these studies that CD4 and CD8 can both function as signalling molecules, but that they are not functionally identical molecules that merely differ in their ligand specificity for MHC class I or MHC class II determinants.